The invention pertains to transceivers and modems. More particularly, the invention pertains to asymmetric digital subscriber line (ADSL) modems used to achieve very high speed data communication via telephone networks.
There is an ever present desire to maximize the speed of digital communications via networks, and particularly telecommunication networks, such as public telephone systems. Accordingly, telecommunication network providers now offer to their customers many options for coupling to the telephone network in addition to the standard analog based connection commonly referred to as POTS (Plain Old Telephone System). Some of the options that are widely available are integrated services digital network (ISDN), T-1 lines, E-1 lines, digital subscriber lines (DSL) and asymmetric digital subscriber lines (ADSL). ADSL""s can provide very high data speeds such as on the order of several megabits per second, over a standard twisted wire pair. Unlike the traditional data modems used for analog communication with a telephone central office via a twisted wire pair, ADSL requires modems both at the subscriber end and at the telephone company Central Office end. Current ADSL systems employ discrete multitone (DMT) technology to implement high bandwidth communications, such as for digital TV broadcast, on demand video, high speed video based internet access, work at home digital file transfer, teleconferencing, home shopping, and information services over existing twisted wire pair telephone lines.
The international telecommunications union (ITU) has promulgated a standard for ADSL that is commonly termed G.lite and which is set forth in ITU-T specification G.992.2, incorporated herein by reference. Another standard promulgated by ANSI is commonly termed Heavy ADSL and is set forth in ANSI specification T1.413, issue 2, also incorporated herein by reference. G.lite has 256 samples with 128 tones (32 tones for upstream communications), each tone having a real and imaginary portion. G.lite uses a cyclic prefix of length 16 samples. Heavy ADSL has 512 samples with 256 tones (32 tones for upstream communications), each tone having a real and imaginary portion. Heavy ADSL utilizes a cyclic prefix of length 32 samples.
FIG. 1A is a block diagram of the basic ADSL modem functions in accordance with ITU-T G.992.2 with 256 samples per symbol. The upper half of the diagram represents functions in the transmit direction while the lower half represents functions in the receive direction.
In the transmit direction, digital data is generated in block 102. That data is processed through a scrambler 104 and then through a forward error correction (FEC) encoder 106 which adds syndrome bytes to the data that will be used for error correction by the receiver at the receiving terminal. Next, as shown in block 108, the transmit data is encoded using quadrature amplitude modulation (QAM). The data is then converted from the frequency domain to the time domain via inverse fast fourier transform (IFFT) 110.
A 1:4 interpolator 112 interpolates the output of IFFT block 110 to produce 256 samples from the 64 samples output from block 110. A cyclic prefix is added to each frame in block 114. The cyclic prefix comprises data added between the symbols to avoid inter symbol interference (ISI). The cyclic prefix is of a standardized length, e.g., 16 samples for ITU-T G922.2 or 32 samples for ANSI-T1.413, issue 2, and is removed at the receiver to recreate the original transmitted data. The data is then forwarded to a coder/decoder (CODEC) 116. The CODEC encodes the data for transmission over the twisted wire pair to the receiving device.
In the receiver portion of the transceiver 100, the received signal is passed from the twisted wire pair through the CODEC 116 where it is decoded. It is then passed to a time domain equalizer (TEQ) 118 to shorten the channel impulse response. Then, in 120, the cyclic prefix is removed. Next, an echo canceller (EC) 134 creates an echo cancellation signal based on the transmit signal which is subtracted by subtractor 121 from the receive signal in order to cancel any echo of the transmit signal that might return over the twisted wire pair and interfere with the receive signal. The echo compensated signal is converted back to the frequency domain by fast fourier transform (FFT) in 122. Then, in 124, frequency domain equalization (FEQ) is employed to compensate for the channel distortion and inter symbol interference (ISI). The receive signal is then processed through a quadrature amplitude modulation decoder (QAM) 126 to decode the tone signal into digital data. That is followed by forward error correction (FEC) 128 which uses the syndrome bits that are added by the transmit path FEC encoder 106 to perform forward error correction. Finally, the data is descrambled to extract the true data signal in 130 and then forwarded to the receiver 132.
FIG. 1B is a block diagram of the basic modem functions in accordance with ANSI-T1.413, issue 2. Note that the functions are similar to those for ITU-T G.992.2. Notable differences include that the receive path FFT 122a is a 512 (rather than 256) point FFT and the transmit path interpolator 112a is a 1:8 interpolator (rather than a 1:4 interpolator). Both differences are a result of the different number of samples between the two protocols.
FIG. 2 is a block diagram which helps illustrate time domain equalization in DMT in more detail. In particular, block 202 encompasses functions performed at the transmitter. Block 207 encompasses functions that occur in the data channel and block 212 encompasses functions performed at the receiver. Block 206 encompasses those functions which result in channel shortening. Thus, it can be seen that, in the transmitter 202, a DMT signal, x(n), is generated for transmission via the channel. The channel 207 has a channel impulse response, h(n), represented by block 208. Accordingly, the signal, y(n), received at the receiver 212 is the convolution of the original signal x(n) and the channel response h(n), i.e.;
y(n)=x(n)*h(n)xe2x80x83xe2x80x83Eq. (1)
The time domain equalization circuit 118 in the receiver 212 convolves the received signal, y(n), with a coefficient 1+a(n). The output yxe2x80x2(n) from the time domain equalization domain circuit 118 therefore is:
yxe2x80x2(n)=y(n)*(1+a(n))=x(n)*(h(n)*(1+a(n)))xe2x80x83xe2x80x83Eq. (2)
The TEQ circuit 118 shortens the channel by the length of the cyclic prefix, e.g., 16 samples for G.lite or 32 samples for Heavy ADSL. Then the cyclic prefix is removed from the signal yxe2x80x2(n), as shown in block 120 in FIG. 1. If we call hxe2x80x2(n) the channel impulse response corresponding to the shortened channel, then hxe2x80x2(n)=h(n)*(1+a(n)).
If the impulse response length of the channel after shortening (hereinafter the shortened channel length, M) is less than or equal to the length, L, of the cyclic prefix, i.e., Mxe2x89xa6L, then each yxe2x80x2(n) will have no dependency on any transmitted samples before x(nxe2x88x92L).
After the cyclic prefix is removed, the actual symbol (without inter symbol interference) should remain. However, if Mxe2x89xa6L, then the sampling signal y(n) is dependent on the transmit sampling signal that appears before x(nxe2x88x92L) and, thus, the removal of the cyclic prefix would not prevent inter symbol interference and thus degrade the performance of the transceiver. Hence, it is desirable to design an efficient TEQ shortening filter, 1+a(n), that makes Mxe2x89xa6L and for which the energy in the channel impulse response hxe2x80x2(n) is as small as possible for nxe2x89xa7L and as large as possible for n less than L.
The invention is a method and apparatus for determining the coefficients to be used by a time domain equalization circuit in a receiver in a communication system using a discrete multitone (DMT) protocol. The method and apparatus is particularly efficient in that the order of the channel response equation after channel shortening is less than or equal to the allotted cyclic prefix length utilized in the communication protocol. Further, the invention includes an efficient method and apparatus for determining symbol synchronization for the time domain equalization.
More particularly, the invention is a method and apparatus for generating coefficients for performing time domain equalization on a digital multitone signal involving (1) synchronizing the received DMT signal to a corresponding signal that was transmitted from said transmitter prior to inclusion of any channel response, (2) determining time domain equalization coefficients via
(RxyTRxxxe2x88x921Rxyxe2x88x92Ryy)aopt=r2xe2x88x92(RxyTRxxxe2x88x921r1)
or
aopt=(RxyTRxxxe2x88x921Rxyxe2x88x92Ryy)xe2x88x921(r2xe2x88x92RxyTRxxxe2x88x921r1).
where
Rxx(k)=E{x(n)x(n+k)}=Rxx(xe2x88x92k)
Rxy(k)=E{x(n)y(n+k)}=Ryx(xe2x88x92k)
Ryy(k)=E{y(n)y(n+k)}=Ryy(xe2x88x92k)
      r    =                  E        ⁢                  {                                    y              ⁡                              (                n                )                                      ⁢                          W              ⁡                              (                n                )                                              }                    =              [                                                            r                1                                                                                        r                2                                                    ]              ;
and
T represents a transpose function; and
(3) time domain equalizing the received signal using said determined coefficients.